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Mechanical Engineering Department

Mechanics of SolidsLaboratory (MECH 321) ( AERO 321)

Spring 2014

Submitted on: 18thof March, 2014

Abeer Al Hammadi 100036374

Amna Al Dhanhani 1000

Fatima Al Hammadi 100036636

Laila Al Dhaheri 100036386

Maitha Al Hashemi 1000

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Abstract:

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Introduction:

Engineers are called upon to design and produce a variety of objects and structures. The

designing process requires an intensive understanding of the materials chosen. Practically, it is acommon practice to test most materials before they are accepted for processing, and before they

are put into service. This is to determine whether they meet the specifications required for the

design or not. Products such as the strain gage that measure fatigues in metals play vital roles in

productivity and safety. A strain gage is a resistance-based sensor used mainly by mechanical

engineers. Specifically, strain gages are in common use in the industry; Engineers measure stress

in mechanical components using a variety of means. Strain gages can be a part of load cell or

torque sensor to directly measure force, pressure or position.

A strain gauge can be used in four different connections: Quarter Bridge Connection, Opposite

Arms Half Bridge Connection, Adjacent Arms Half Bridge Connection, and Full Bridge

Connection. Table.1 summarizes the characteristics of the three connections:

Table 1: Comparison between the different connections

CharacteristicsQuarter Bridge

ConnectionOpposite Arms HalfBridge Connection

Adjacent Arms HalfBridge Connection

Full Bridge Connection

Number of straingauge connected to

the display strainunit

Single strain GageTwo strain Gage (in

opposite direction)

Two strain Gage (in

adjacent direction)Four strain Gage

Number of dummyplugs

Three dummy plugs Two dummy plugs Two dummy plugs One dummy plugs

Number of ActiveArms

One active arm Two active arm Two active arm Four active arm

Connections

Schematic

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1. Description of Experimental Setup

Figure.1 is showing the major parts in the experimental panel setup.

Main Part

Panel

Weight hangerOutputBeam

Output to Display

Unit

Figure 1: Experimental panel setup

Bending system

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The digital strain display unit is shown in Figure.2 . It is typically a metal box with a display, controls and

input sockets. To begin with, there is a button that is used to allow the user to zero readings before each

experiment. Also, there are four input sockets in order to change the bridge connections to obtain different

readings. The gauge factor control allows the user to set the gauge factor of the strain gauge that they

connect to (fixed at 2.11). Moreover, a configuration switch is used to show the correct strain for any

bridge connection, as this is accomplished by changing calculations of strain display. The digital display

shows the output voltage measured across the bridge and automatically calculates and displays the strain.

Figure 2: Digital strain display unit

Digital

display

Output

VDAS

Press and

Hold to

Zero

Button

Input

socketsGauge

factor

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Results and Analysis

Qustion1:Calculate the strain values using the equation supplied previously and the bridge

output voltages corresponding to the number of active arms. This information can be found in the

appendix. Confirm that the calculated value of strain is the same as that displayed by the Strain

Display.

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Question2: Compare the output voltage and strain reading for each bridge configuration.

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Question4: Calculate the gradient of each line and compare the quarter, half and full bridge

outputs. Do these compare well and prove the Wheat Stone Bridge theory?

Connection Gradient of the line

Quarter Bridge

Half Bridgeopposite

Half Bridgeadjacent

Full Bridge

Table 1: Gradient calculations of each line

The gradient was found for different connections as shown in Table 6. Moreover, the procedure in finding

the gradient can be found in third question section.

Figure 4: Wheatstone bridge circuit

Because of its outstanding sensitivity, the Wheatstone bridge circuit (depicted in figure above) is the most

frequently used circuit for static strain measurement. VIN is the input voltage to the bridge, Rg is the

resistance of the strain gage, R1, R2and R3are the resistances of the bridge completion resistors, and VOUT

is the bridge output voltage. For the Half Bridge strain gauge circuit, voltage sensitivity to strain is twice

that of Quarter Bridge. For the Full Bridge strain gauge circuit, voltage sensitivity four times the

sensitivity of Quarter Bridge. Thus, the results we found in the experiment compare well to Wheat Stone

Bridge theory since the gradient of Half Bridge connection is twice that of Quarter Bridge connection andthe gradient of Full Bridge connection is twice that of Half Bridge and 4 times that of Quarter Bridge. [1]

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Conclusion

There are different techniques that can be used to measure strain, in this experiment a strain gage

is used to determine the deformation on a metallic beam. Different ways of connecting were

used: Quarter Bridge, half bridges and full bridge connections to obtain the strain readings. Onevalue of the strain which was calculated using the equation provided in the appendix and the

other was directly displayed on the strain digital display unit (Versatile Data Acquisition Unit).

Comparing these values of strain for the quarter bridge connection and the full bridge

connection, the results of the calculated and the actual are almost the same and the a percentage

error are includes in the tables for the three types of connections (quarter bridge, half bridges and

full bridge).

This error might be a result of lack of consistency when performing the connections, or the wires

were not connected well. Also, the placements of the loads at the given positions are not

accurate; in addition, miscounting the number of the loads is another reason. Overall, lack of

experience is the main source of error. With more experience, we would know what sources of

errors are possible to occur, so there would be a chance to think of ways to reduce them before

starting the experiment.

After performing this experiment, we learned how to use strain gages to measure strain of a

given specimen using different connections.